In, for example, data- and telecommunications, it frequently happens that information has to be transmitted over a channel that is subject to disturbances of some sort. If the information is not successfully received at a recipient, the information may have to be retransmitted. This is of course something that preferably is to be avoided, if communications resources are to be efficiently used. To improve the likelihood of successful reception of information, various techniques have been developed. For example, use of appropriate selection of access or modulation scheme, channel coding, interleaving etc.
The above applies in particular to wireless communications, where communications are performed over radio channels, since there are many ways for the radio channel to be disturbed, e.g. by various forms of fading, by interference, by noise etc.
In, for example, a GSM system (Global System for Mobile Communications), information is transmitted between a Mobile Station (MS) and Base Station (BS) using radio signals. Several logical channels are defined in order to support transmissions of different types of information. In general, the logical channels are divided into traffic channels and control channels. Traffic channels carry user data, e.g. speech, and control channels carry information used by the system, e.g. for setting up calls. Each logical channel is coded and modulated in a transmitter and sent using a radio signal. The coding involves channel coding, e.g. convolutional codes, and interleaving to protect the information from the disturbances influencing the radio channel. At a receiver, the radio signal is demodulated and then decoded for each logical channel.
For proper operation, it is important to balance a design of the channel encoding and interleaving between different channels. For example, if the coding of the control channels is too weak, then calls might be dropped due to bad reception even if e.g. the speech is decoded correctly. This was, for example, seen in the GSM system when Adaptive Multi Rate (AMR) speech was introduced. AMR is an improved coding of speech, which increases capacity and/or coverage. When the speech coding was improved, the control channels became a bottleneck of the system.
A particular difficulty occurs when responsibility for a connection to a mobile station is to be switched from a currently serving base station to new serving base station, a process known as handover or hand-off. When the mobile station is about to loose the connection to the currently serving base station in one cell, the connection must be quickly transferred the new serving base station in a neighbouring cell. The communications needed to achieve handover is sent on the control channels. First, a Base Station Controller (BSC) is continuously notified about the reception characteristics of the current cell and neighbouring cells using measurement reports. At some point in time, the BSC decides that a handover should be made and sends a handover command via the serving base station to the mobile station. The mobile station then replies with an acknowledgement. The handover command is often so long that it has to be sent in several frames, and each successfully received frame must then be acknowledged. If the serving base station fails to decode a sent acknowledgement, an unnecessary retransmission is effected. This extends the duration of the handover process and therefore increases a probability that the connection will be lost.
In later versions of the GSM standard some improvements have been made to the control channels (“Mobile Station-Base Station System (MS-BSS) interface; Data Link (DL) layer specification”, 3GPP TS 44.006, V6.3.0). For the handover commands, the mobile station can combine original transmissions with retransmissions in order improve a probability of successful decoding of the handover command. The same applies to the BS with respect to the measurement reports from the mobile station. However, the above-mentioned difficulty with the acknowledgements still remains.